1. Field of the Invention
The present invention relates generally to liquid redox processes for the removal of sulfur from gases, such as natural gas, or from tail gases from a sulfur plant.
2. The Prior Art
Hydrogen sulfide (H.sub.2 S) is often encountered in gas streams, such as natural gas being extracted from the ground, or in gases being produced, either intentionally or unintentionally in various industrial processes, such as the tail gas from Claus sulfur plants. In the presence of oxygen, hydrogen sulfide can form various oxides which are not only pollutants (which can contribute to acid rain formation), but also which can be corrosive or otherwise damaging to equipment, such as pipelines and other machinery.
Processes for the removal of hydrogen sulfide from gas streams are known. A basic agent such as an amine absorbent, may be employed, in which case the amine is regenerated for reuse as an absorbent by heat, for example, by steam. A variety of non-regenerable processes using iron-based solids, liquid-based processes using caustic triazine have also been employed. For economic reasons, the regenerable processes such as those that employ amines and liquid oxidation-reduction (redox) solutions are more attractive as the total amount of sulfur that must be removed increases, to the range of 50-100 pounds per day. When conversion to elemental sulfur is desired for environmental or regulatory reasons, liquid redox processes are preferred when total sulfur that must be removed from the gas stream is on the order of 10 tons per day.
Generally, in a typical liquid redox process, an oxidation-reduction system is used in which the hydrogen sulfide-laden gas ("sour" gas) is exposed to a sulfide precipitation catalyst material (for example, a metal oxide, in which the metal cation changes from a higher valence state to a lower state, upon reaction with the hydrogen sulfide), and the gas, now with a substantially reduced level of hydrogen sulfide ("sweet" gas) is then piped onward to its intended destination.
After passing through the absorber, at least a portion of the sulfur will have precipitated out of the precipitation catalyst solution as elemental sulfur. The precipitation catalyst solution is then sent to some form of regeneration apparatus, such as an oxidizer, for restoration of the metal cation in the precipitation catalyst solution to the desired higher valence state, so that the solution may be returned to the absorber to absorb more hydrogen sulfide from the gas stream.
Such liquid redox processes are favored since they operate at ambient temperatures and have high selectivity for hydrogen sulfide. While one of the major attributes of liquid redox processes for removing hydrogen sulfide from sub-quality natural gas is the rapid reaction rate of the hydrogen sulfide with the liquid redox solution and the subsequent precipitation of elemental sulfur, the process is marred by the tendency of the elemental sulfur to deposit on internal surfaces of the absorber such as walls, static mixers, packing and so on. The tendency for sulfur deposition primarily emanates from the fact that there is always a zone of stagnant fluid associated with a fixed surface. That is, a thin "boundary layer" of non-moving liquid is present on non-moving surfaces. As a result, the sulfur precipitating from the liquid onto the non-moving surfaces is essentially continuous and eventually clogs the absorber. The absorber must then be taken out of service and cleaned, resulting in plant downtime and economic penalty to the user. One method which has been considered to keep such surfaces free of sulfur deposits would be to maintain a large flow of recirculated liquid through the absorber with turbulence to ensure continuous scrubbing of the stagnant zones within the absorber. Such a method would not have a high degree of certainty in achieving uniform turbulence throughout the absorber. In addition, such a method could be costly in terms of the additional amounts of absorbent which must be maintained in the circulation loop, and the energy and equipment costs associated with constant circulation and turbulence.
It would therefore be desirable to provide a method for assuring the prevention of buildup of elemental sulfur on non-moving surfaces within an absorber, without having to resort to continuous circulation of absorbent within the absorber.